1. Field of the Invention
The present invention relates to a controller for controlling switching elements of a power supplying apparatus for supplying power to a load such as a three-phase motor through the switching elements.
2. Description of the Prior Art
FIG. 7 shows a circuit diagram of the main circuit of a power supplying apparatus controlled by a prior art controller of power supplying apparatus. Referring to FIG. 7, reference numeral 1 denotes a three-phase motor as a load. Reference numeral 2 denotes a DC power source. Reference numeral 3 denotes the switching element on the positive-phase side of the U-phase circuit. Reference numeral 4 denotes the switching element on the negative-phase side of the U-phase circuit. Reference numeral 5 denotes the switching element on the positive-phase side of the V-phase circuit. Reference numeral 6 denotes the switching element on the negative-phase side of the V-phase circuit. Reference numeral 7 denotes the switching element on the positive-phase side of the W-phase circuit. Reference numeral 8 denotes the switching element on the negative-phase side of the W-phase circuit. The switching elements 3 to 8 are rendered conductive when the gate voltage is at an L level. Reference numeral 9 denotes an interface circuit responsive to control signals from a controller 10 for controlling the switching elements 3 to 8. Reference numeral 10 denotes the controller.
FIG. 8 is a block diagram showing structure of the controller 10. Referring to FIG. 8, reference numerals 11u, 11v, and 11w denote timers. The timer 11u, upon receipt of a control command for the switching elements 3 and 4 from a speed controlling circuit, not shown, starts counting time and, at the same time, outputs a signal at an H level. Thereafter, it, when the counted time reaches a preset period of time T1, outputs a signal at an L level. The timers 11v and 11w perform the same functions as the timer 11u does, except that the control object are switching elements 5 and 6 and switching elements 7 and 8, respectively. Reference numerals 12u, 12v, and 12w denote short-circuit preventing timers. The short-circuit preventing timers 12u, 12v, and 12w, when the signal levels of the signals outputted from the timers 11u, 11v, and 11w change, respectively, from H level to L level, output a pulse signal with a pulse width of T2 (T2 corresponds to the short-circuit preventing period of time). Reference numerals 13u, 13v, and 13w denote flip-flops. The flip-flops 13u, 13v, and 13w, when the signal level of the signal outputted from the timers 11u, 11v, and 11w, respectively, changes from H level to L level, invert the signal level of the signal outputted therefrom.
Further, reference numerals 14u, 14v, and 14w denote flip-flops. The flip-flops 14u, 14v, and 14w, when the signal level of the signal outputted from the short-circuit preventing timers 12u, 12v, and 12w, respectively, changes from H level to L level, output a signal having a different signal level from the signal level of the signal outputted from the Q terminal of the flip-flops 13u, 13v, and 13w. Reference numerals 15u, 15v, and 15w denote NAND circuits. To one input terminals of the NAND circuits 15u, 15v, and 15w, the signals outputted from the flip-flops 14u, 14v, and 14w are inputted, respectively, and to the other input terminals, the signals outputted from the Q terminals of the flip-flops 13u, 13v, and 13w are inputted, respectively. Reference numerals 16u, 16v, and 16w denote flip-flops. The flip-flops 16u, 16v, and 16w, when the signal level of the signal outputted from the short-circuit preventing timers 12u, 12v, and 12w changes from H level to L level, respectively, output a signal having a signal level different from the signal level of the signal outputted from the QB terminal of the flip-flops 13u, 13v, and 13w. Reference numerals 17u, 17v, and 17w denote NAND circuits. To one input terminals of the NAND circuits 17u, 17v, and 17w, the signals outputted from the flip-flops 16u, 16v, and 16w are inputted, respectively, and to the other input terminals, the signals outputted from the QB terminals of the flip-flops 13u, 13v, and 13w are inputted, respectively. Reference numerals 18u, 18v, and 18w denote terminals on the positive-phase side of the U-phase, V-phase, and W-phase, respectively. Reference numerals 19u, 19v, and 19w denote the terminals on the negative-phase side of the U-phase, V-phase, and W-phase, respectively.
Now, operations will be described. Since the operations of the V-phase and W-phase are the same as the operations of the U-phase, description will only be given on the operations of the U-phase and description on the operations of the V-phase and W-phase will be omitted.
First, when driving a three-phase motor 1, it is necessary to supply AC power to the three-phase motor 1. Especially when it is required to control the revolving speed of the three-phase motor 1 to change over a wide range, sometimes, a power supplying apparatus in which AC power is once converted to DC power and then the DC power is inverted to desired AC power and supplied to the three-phase motor 1 is used (in FIG. 7, the portion converting AC power to DC power is omitted).
In the use of such a power supplying apparatus for supplying power by once converting AC power to DC power and then converting the DC power back to AC power, it becomes possible to supply desired AC power to the three-phase motor 1 by suitably controlling the firing angles of the switching elements 3 to 8. However, when the switching element 3 on the positive-phase side and the switching element 4 on the negative-phase side of the U-phase are rendered conductive at the same time, for example, it short-circuits the DC power source 2 in a no-load condition and, hence, it sometimes occurs that a large current flows from the DC power source 2 to ground through the switching elements 3 and 4 and, thereby the switching elements 3, 4 and the like are damaged.
Therefore, in order to prevent the switching element 3 on the positive-phase side and the switching element 4 on the negative-phase side from becoming conductive at the same time, first, when the timer 11U receives a control command for the switching elements 3 and 4 from a speed control circuit, not shown, at the point of time A as shown in FIG. 9, it starts counting time and, at the same time, outputs a signal at an H level and, when the counted time reaches a preset period of time T1, it outputs a signal at an L level (in the example of FIG. 9, the counted time reaches the preset period of time T1 at the point of time B).
When the output signal from the timer 11u changes from H level to L level at the point of time B, the short-circuit preventing timer 12u is triggered by the trailing edge and outputs a pulse signal with a pulse width corresponding to a short-circuit preventing period of time T2.
The flip-flop 13u is also triggered by the trailing edge of the output signal of the timer 11u and inverts the signal levels of its output signals (in the example of FIG. 9, the signal level of the output signal from the Q terminal is changed from L level to H level and the signal level of the output signal from the QB terminal is changed from H level to L level).
Further, since the signal level of the output signal from the QB terminal of the flip-flop 13u changes from H level to L level at the point of time B, the NAND circuit 17u is triggered by the trailing edge and changes the signal level of its output signal from L level to H level.
Thus, a signal at an H level is outputted from the terminal 19u on the negative phase side of the U-phase to the interface circuit 9 at the point of time B, and hence, the switching element 4, which has been in the conductive state from the time before the point of time A, is brought into the non-conductive state at the point of time B.
When the signal level of the pulse signal, which was outputted from the short-circuit preventing timer 12u at the point of time B, changes from H level to L level at the point of time C, the flip-flop 14u is triggered at its trailing edge and changes the signal level of its output signal from H level to L level.
Further, at the point of time C, the flip-flop 16u is also triggered by the trailing edge and changes the signal level of its output signal from L level to H level.
Then, when the signal level of the output signal from the flip-flop 14u is changed from H level to L level at the point of time C, the NAND circuit 15u is triggered by its trailing edge and changes the signal level of its output signal from H level to L level.
Thus, at the point of time C, a signal at an L level is outputted from the terminal 18u on the positive side of the U-phase to the interface circuit 9, and, hence, the switching element 3 which has been in the non-conductive state from the time before the point of time A is brought into the conductive state at the point of time C.
Thereafter, each time the timer 11u receives the control command for the switching elements 3 and 4 from the speed control circuit, not shown, the switching states of the switching elements 3 and 4 are controlled according to the same principle. At this time, as apparent from FIG. 9, both the switching element 3 and the switching element 4 are in the non-conductive state during the short-circuit preventing period of time T2, and, hence, the short-circuiting of the power supply (the state in which both the switching element 3 and the switching element 4 are rendered conductive at the same time), which is possible to occur when the switching states are switched over, can be prevented.
Since the prior art controller of power supplying apparatus was constructed as described above, the short-circuiting of the power supply possible to occur when the switching element 3 and the like are controlled can be prevented. However, since there is provided no means for suppressing a large transient current flowing from the power source 2 to the coil of the three-phase motor 1 through the switching element 3 and the like when the switching element 3 and the like in the non-conductive state is put into the conductive state, there have been such problems as increase of current consumed by the three-phase motor 1.
Incidentally, technologies to control the current flowing into the coil of a three-phase motor 1 by controlling the conducting period of time of the switching element 3 and the like are disclosed in gazettes of Japanese Patent Laid-open No. Hei 1-99493 and Japanese Patent Laid-open No. Hei 6-38539. In these gazettes, however, no disclosure is made as to the technology to suppress the transient current occurring when the switching element 3 and the like in the non-conductive state are put into the conductive state.